Installing a casing in a borehole of an oil or natural gas well is an important part of the drilling and completion process. The casing serves to strengthen the surface of the borehole and ensures that no oil or natural gas s seeps out of the well as it is brought to the surface, and further ensures that other fluids or gases do not seep into the formation through the borehole. In particular, the casing prevents losses of drilling fluid circulating down the borehole through a drill pipe string and a drill bit carried on the downhole end of the drill pipe string and further circulating upward to the top of the o borehole through an annulus between the drill pipe string and the wall of the borehole. The drilling fluid cools the drill bit, removes cuttings from the borehole and maintains hydrostatic pressure on pressurized subterranean formations.
Usually, the surface or wall of the borehole is stabilized by running and cementing a tubular casing into the borehole, which means that drilling the borehole normally is a sequential process in which drilling the borehole and installing the casing alternate. The process is time-consuming, since the drill pipe string has to be removed from the borehole for installing the casing.
It is known to use the tubular casing instead of the drill pipe string to direct and rotate the drill bit. In such a casing while drilling system, the casing is part of the drilling assembly and may be cemented in place where the appropriate depth is reached, and thereafter a length of the tubular casing is run through the cemented casing portion for further drilling the borehole. The casing while drilling process is unpredictable to some extent, since the casing quite easily may stick to the borehole, which makes the position of the casing shoe unpredictable, and some length of the casing may be lost with the result that the well may not reach desired depth (Nediljka Gaurina-Medimurec, “Casing Drilling Technology”, Rudarsko-geolosko-naftni zbornik, Zagreb 2005, Vol. 17, pages 19 to 26).
From U.S. Pat. No. 7,334,637 B2 it is known to form a temporary liner in a wellbore by extruding a fusible polymer liner material, such as polyethylene or polypropylene from an assembly supported on the drill pipe string. An extruder extrudes the liner material onto the wall of the borehole while the liner material is fed from a reservoir at the surface level of the borehole through an additional piping running through the drill string. A heat source, for example a laser device, melts the fusible liner material extruded onto the wall of the borehole to produce the liner.
The liner produced according to U.S. Pat. No. 7,334,637 B2 is a temporary liner intended to be replaced later on by a conventional tubular casing to be cemented in the borehole. The system requires an additional piping through the drill pipe. The fusible liner material must be capable of being extruded onto and adhered on the wall of the borehole.
Another method for stabilizing a wellbore during drilling in a sequential process is known from U.S. Pat. No. 5,944,105. A downhole portion of the drill pipe string is provided with a plurality of nozzles through which fluid jets can be ejected. After having drilled the borehole into an unstable subterranean formation, fluid is pumped through the nozzles to enlarge the borehole by fluid jet erosion while moving the drill pipe string upwardly. After having enlarged the diameter of the borehole, a hardenable, permeable material, for example a hardenable organic resin, is ejected through the nozzles to fill the enlarged portion of the borehole. The material is caused to harden by heat or a hardening agent, and thereafter the borehole is redrilled through the hardened material. The known method does not allow a continuous lining of the formation while drilling.
From WO 2005/121 198 A1 another sequential process for in-situ stabilizing the wall of a wellbore is known. After having drilled the borehole through a weak formation, the drill string is pulled up above the weak interval to be stabilized. A resin mixture is pumped through the drill string into the borehole to displace the drilling fluid from the drill string and the annulus between the drill string and the wall of the borehole and to squeeze the resin into the weak formation. After squeezing resin into the formation, the well is shut for several hours prior to cleaning set resin out of the wellbore and resuming drilling operation to deepen the well.
From U.S. Pat. No. 6,311,773 B1 it is known to consolidate particulate solids in subterranean zones around a wellbore by causing a hardenable resin composition to flow between the particulate solids of the subterranean zone. By hardening the resin composition, the particulate solids will be consolidated into a hard, permeable pack. Similar methods for consolidating the wall of a borehole are known, for example, from EP 0 879 935 A2, U.S. Pat. Nos. 7,216,711 B2, 7,264,052 B2, WO 03/102 086 A2, EP 0 542 397 A2 or U.S. Pat. No. 4,428,426. These documents disclose resin-coated particles, for example sand grains or other proppants, for treating subterranean formations, in particular subterranean fractures.
It is the main object of the invention to provide an equipment and a method, which allows reinforcing a borehole of a well, in particular a well of petroleum and/or natural gas, while at the same time drilling the borehole.
The equipment for drilling and reinforcing the borehole basically comprises equipment as it is known from U.S. Pat. No. 7,334,637 B2, in particular:                a drill pipe string carrying a drill tool at its lowermost end;        drilling fluid circulation means for circulating drilling fluid through the drill pipe string and an annulus between the drill pipe string and the borehole and        a downhole treatment device held on the drill pipe string for applying a liner of polymer material at the surface of the borehole.        
The improvement according to the invention is characterized in that the drilling fluid circulated through the annulus is a fluid system containing fusible and/or curable polymer material dissolved and/or emulsified and/or dispersed therein, and the treatment device is adapted to concentrate energy for fusing and/or curing the polymer material contained in the drilling fluid in a limited space at the vicinity of the surface of the borehole.
Since the polymer material for producing the liner of the borehole is contained in the drilling fluid (mud) anyway needed for drilling the borehole, no additional piping along the borehole or no downhole reservoir for polymer material is needed. It is an essential feature of the invention that the treatment device confines the energy necessary for forming the liner at the surface of the borehole to a limited space adjacent the wall so as not to fuse or cure polymer material within the bulk (rest) of drilling fluid circulating through the drill pipe string and the annulus.
The treatment device is positioned at the drill pipe string downhole in the vicinity of the drill tool, which may be in the form of a simple drill bit, but also may include a roamer or a downhole assembly including a downhole drill motor. Due to the downhole pressure of the drilling fluid, some of the drilling fluid including polymer material may be pressed into the pores of the formation and anchors the liner to the wall of the borehole. The polymer material may be dissolved or emulsified within the drilling fluid, but in particular is in a particulate form, for example in the form of powder-like particles or granules, which adhere to each other when being fused or cured by energy from the treatment device.
Polymer materials suitable for forming the liner of the borehole are known in the art; reference is made to the patent documents mentioned above. Further suitable polymer material is known from EP 1 664 481 B1, WO 2005/121 500 A1 or WO 02/14 453 A1.
The particulate polymer material may consist of polymer material only. Preferably, the particulate polymer material comprises solid particles coated with fusible and/or curable polymer material to mechanically strengthen the liner formed on the wall of the borehole. In a preferred embodiment, the solid particles are comprised of metal, in particular steel, to provide for ductility and toughness of the liner while the polymer material will bind the composite together.
Preferably, the particles of the particulate polymer material have a diameter of less than 1 mm, preferably of less than 0.3 mm, for example 0.1 mm, to improve anchoring in the formation and to reduce the porosity of the liner. A diameter of less than 0.3 mm is advantageous if the polymer material is coated onto particulate metal cores.
In a preferred embodiment, the treatment device comprises an energy radiating device which produces at least two distinct energy beams which are directed from different positions to a common spot in the limited space in the vicinity of the surface of the borehole where the energy beams intersect and focus the energy within said limited space. While the energy of a single energy beam does not suffice to fuse or cure the polymer material in the bulk of the drilling fluid, the focused energy of the plurality of the energy beams is sufficient for producing the liner.
In another embodiment, the energy source, e.g. an energy output port of the treatment device is positioned near the surface of the borehole and directs its energy beams directly onto the surface. An energy shielding or an energy reflector concentrates the energy to the limited space in which the liner is to be produced and protects the bulk of the drilling fluid outside said limited space from radiated energy.
The energy source of the treatment device can be of any type that allows directed radiation of energy onto the surface of the borehole. Preferably, the energy source is a laser device or an induction heating device or a microwave radiating device or a super-sonic energy radiating device. The type of the energy source will be chosen depending on the polymer material and/or in case of particulate material comprising a core, e.g. a metal core, in dependence of the material of the core, as it known in the art.
The liner is continuously produced on the wall of the borehole. The thickness can be controlled by controlling the concentration of the polymer material within the drilling fluid, the axial of the speed of the drill pipe string and the circulating velocity of the drilling fluid along the wall of the borehole. Depending on the porosity of the formation, the polymer material may migrate into the formation to seal and/or improve anchoring of the liner at the formation. Basically, it is sufficient to compact the polymer material contained in the drilling fluid starting from the average concentration of the polymer material in the drilling fluid, but preferably the treatment device is adapted to specifically raise the concentration of the polymer material in the vicinity of the wall and in particular in the vicinity of the limited space, in which the treatment device concentrates energy for fusing and/or curing the polymer material.
In a preferred embodiment, additional pressure is exerted onto particulate polymer material by magnetic forces produced by at least one magnet of the treatment device. The particulate polymer material comprises solid particles of a diamagnetic material, for example copper, which is repelled within the magnetic field produced by the treatment device onto the surface of the borehole. The magnetic repellent force pushes the particles towards and into the formation where the particles concentrate for forming the liner.
If the drilling fluid contains particulate polymer material comprising solid particles having a particle density higher than the density of the drilling fluid including particulate material other than the particulate polymer material, the concentration of the polymer material in the vicinity of the wall of the borehole can be raised by a centrifugal separator coaxially arranged with the drill pipe string. The centrifugal separator centrifugates the higher density particulate polymer material towards the wall of the borehole while the drilling fluid flows axially along the annulus. The centrifugal induces a whirl in the drilling fluid around the drill string a certain distance before and in the limited space curing position. Preferably, the solid particles of the particulate polymer material have a density which is higher than the density of formation particles contained in the drilling fluid and also higher than the density of the rest of the drilling fluid. Due to the centrifugal action the particles with the highest density, e.g. the particulate polymer material will be separated onto the wall of the borehole to produce the layer while lighter components of the drilling fluid will remain in a radially inner portion of the annulus.
In a preferred embodiment, the centrifugal separator is in the form of a helical vane coaxially stationary surrounding the drill pipe string. In another embodiment, the centrifugal separator can be in the form of a motor-driven impeller coaxially rotating with respect to the drill pipe string. The impeller has a fan wheel which produces the whirl in the drilling fluid to centrifugate the particles onto the wall of the borehole.
The idea of concentrating particulate polymer material at the wall of the borehole by means of centrifugating the drilling fluid in the annulus can also be carried out with a treatment device not being adapted to concentrate the energy in a limited space near the wall of the borehole. The aspect of the centrifugal separator thus can be used with an equipment the treatment device of which heats the drilling fluid within the total radial depth of the annulus.
The invention further is directed to a method for reinforcing a borehole of a well while drilling the borehole. The method comprises the steps of circulating a drilling fluid containing fusible and/or curable polymer material dissolved and/or emulsified and/or dispersed therein along the surface of the borehole and concentrating energy for fusing and/or curing the polymer material in a limited space in the vicinity of the surface of the borehole while drilling of the borehole is continued. The equipment explained above is intended to carry out the method.